IRVING, Texas – At least two people were killed and more than 40 were hospitalized after a charter bus careened off a North Texas highway and flipped onto its side Thursday, drawing a large emergency response as rescue crews struggled to reach victims inside, authorities said.

The Cardinal Coach Line bus was carrying mostly senior citizens when it suddenly weaved across the busy highway, striking two concrete barriers, and toppled over in the center median, witnesses said. The wreck occurred along President George Bush Turnpike in Irving, just east of Dallas/Fort Worth International Airport.

"It was pretty bad, people screaming," said Ed Cluck, who stopped after driving by the wreck and seeing smoke. He said he popped the bus' roof hatches and helped six to eight people escape.

"It was just people stacked on top of each other," he said.

"It's just a lot of injuries, a lot of people in shock, broken bones," added Robert Hare, another motorist who stopped to help. He said many passengers were crying and appeared to be in shock as they were pulled from the wreckage.

The bus, which was carrying about 45 people, was on its way to a casino in Oklahoma, authorities said.

Texas Department of Public Safety Trooper Lonny Haschel confirmed that two people were killed in the accident. Authorities said 41 other people were taken to local hospitals, many of them suffering from fractured bones.

Emergency vehicles were seen swarming the bus as it lay in the grassy center median, and ladders were being used to access the vehicle. The cause of the accident has not been determined, but the National Transportation Safety Board said it was sending investigators to the scene.

Cardinal Coach has reported no accidents in the last two years that resulted in deaths or injuries, according to the Federal Motor Carrier Safety Administration. A man who answered the phone at the company's offices in Mansfield, just south of Dallas, confirmed that one of its five buses was involved in the crash but said he didn't have time to talk because he was trying to get information about the crash.

Law enforcement officers were interviewing bus passengers and drivers who witnessed the crash. The wreck occurred near a highway interchange, so traffic was snarled for several miles.

"We ended up swirling and weaving and then ended up on the side," passenger Daniel Risik, 73, told The Dallas Morning News. "People were screaming and hollering, a very traumatic situation to say the least."

"People were piled on top of each other," he said. "It was unbelievable. A lady had pinned me. Rescue got there and started pulling people out of a roof emergency hatch. People were hollering, screaming, there was blood all over the place. It was unbelievable."

He said passengers had been picked up in Fort Worth and other locations in the area.

A spokesman for Baylor Medical Center in Irving said 13 patients arrived at the hospital following the accident. Officials at Las Colinas Medical Center in Irving confirmed that six patients were there.

Another 15 patients were transported to Parkland Memorial Hospital, including the bus driver, and another victim was airlifted in critical condition, hospital officials said. Public transportation buses with Dallas Area Rapid Transit were used to transport some passengers with lesser injuries.

The charter bus was heading to a casino in Durant, Okla., about 95 miles north of Dallas, Choctaw Casinos spokeswoman Arlene Alleman said.

The bus company has been beset with financial problems since its founding in 2007, according to court records. The Internal Revenue Service placed a lien on Cardinal Coach's property in December for nearly $60,000 in unpaid business taxes, and the owners, Matt and Teresa Biran of Mansfield, filed for bankruptcy in December 2008.

George Barnes, the attorney who handled the couple's bankruptcy, said the proceeding was discharged three months later and wasn't related to any issues with the bus company, other than it wasn't generating enough income.

"It had nothing to do with any lawsuits, safety aspects," Barnes said Thursday. "It was just a matter of they had way more debt than they had before. It was just one of those situations where they needed a fresh start and this did it."

Thursday's accident comes as bus safety advocates push for quicker implementation of a new law aimed at making motor coaches safer. Approved in July, the law set deadlines for the U.S. Department of Transportation to implement a series of provisions, though a recent spate of high-profile accidents has prompted some supporters to call for an accelerated timeline.

In a March 21 letter to the agency, U.S. Sen. Sherrod Brown of Ohio pointed to a crash on the Pennsylvania Turnpike involving the Seton Hall University women's lacrosse team, as well as others in California, Oregon, Maine and Missouri. The Democrat also noted that a provision requiring the agency to finalize regulations regarding seat belts on buses within a year had "languished."

Jackie Gillan, president of Washington-based Advocates for Highway and Auto Safety, said Thursday's accident was yet another reason to fast-track the law's provisions, particularly those involving seat belts and other equipment upgrades.

"There's nothing to stop anyone from accelerating those deadlines, and in fact one would think they would in light of these crashes," she said.

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David Warren reported from Dallas. Associated Press writers Nomaan Merchant, Diana Heidgerd and Danny Robbins also contributed to this report from Dallas.